Introduction:
Electrical measurements often come down to either measuring current or measuring voltage. Even if you are measuring frequency, you will be measuring the frequency of a current signal or a voltage signal and you will need to know how to measure either voltage or current. In this short lesson, we will examine those two measurements - starting with measuring voltage. However, first we should note a few common characteristics of the meters you use for those measurements.
Many times you will use a digital multimeter - a DMM - to measure either voltage or current. Actually, a DMM will also usually measure frequency (of a voltage signal) and resistance. You should note the following about typical DMMs.
Polarity is important. Usually the terminals of the DMM will be coded to indicate polarity. Often that polarity is indicated by a red terminal (positive) and a black terminal (negative). In other cases, the polarity could be indicated by printed notes on the terminals.
Often one of the terminals on the DMM may be connected to the ground. That would normally be the black terminal, or it may be indicated with a ground symbol.
Polarity is important. Usually the terminals of the DMM will be coded to indicate polarity. Often that polarity is indicated by a red terminal (positive) and a black terminal (negative). In other cases, the polarity could be indicated by printed notes on the terminals.
Often one of the terminals on the DMM may be connected to the ground. That would normally be the black terminal, or it may be indicated with a ground symbol.
With that in mind, let's get on to measuring voltage.
Measuring Voltage
Voltage is one of the most common quantities measured. That's because many other variables - like temperature, for example - are measured by generating a voltage with a sensor. So, even if you want to measure temperature you might end up having to measure a voltage and convert that reading into the temperature reading you wanted.
Voltage is measured with a voltmeter. However, digital multimeters (DMMs) - which can function as voltmeters - often have considerably more capability and can measure current, resistance and frequency. And, there are other instruments - like oscilloscopes - that measure voltage and should be thought of as voltmeters. No matter what the instrument is, if it measures voltage you have to treat the instrument as a voltmeter.
When you measure voltage you have to remember that voltage is an across variable. When you measure voltage you have to connect the voltmeter to the two points in a circuit where you want to measure voltage. Here is a circuit with a voltmeter connected to measure the voltage across element #4.
Note the following about this measurement.
Notice that the voltmeter measures the voltage across element #4, +V4. (And, the plus sign is important. Remember the polarity issue.)
Notice the polarity definitions for V4, and notice how the red terminal is connected to the "+" end of element #4. If you reversed the leads, by connecting the red lead to the "-" terminal on element #4 and the black lead to the "+" end of element #4, you would be measuring -V4.
And, remember this as well.
When you measure voltage, the voltmeter should not disturb the circuit where you are attempting to measure the voltage. In the circuit above, that disturbance is the current drawn by the voltmeter. You want that current to be as close to zero as it can possibly be. That means that you need to have the resistance of the voltmeter as large as possible. There's more discussion of that effect in the lesson on measuring voltage. Ideally, the resistance of a voltmeter would be infinite.
Measuring Current:
Current is measured with an ammeter. While voltage is a more common measurement, it is often necessary to measure current. When measuring current, it is important to remember that current is a flow variable. Current flows through electrical elements, and if you want to measure current you have to get it to flow through the ammeter. Here's the same circuit we used in the example above. Consider what we would have to do to measure the current flowing through element #4.
Measuring Frequency
When you measure the frequency of a voltage signal, the typical instrument will do the following.
First, the instrument is connected like a voltmeter, and set to measure frequency.
When the measurement is taken, the instrument counts the signal. It might count zero crossings of the signal, or it might just assume that the signal is a sequence of pulses, and count the pulses. In either case, the instrument counts for a predetermined length of time, T (which you might be able to control).
Then, the frequency is computed by dividing the count by the time period, T.
The computation of frequency cannot have a resolution better than one count. For example, if the instrument counts for one second, a count of ten would compute as 10 Hz, and a count of 11 would compute as 11 Hz. You couldn't get a good measurement of 10.5 Hz, and would always be off by 0.5 Hz. What you got would depend upon the timing of the count - when it started.
The resolution is probably not a problem if you are interested in a 20KHz signal and the instrument counts for a second, but you have to be cognizant of what is taking place.
First, the instrument is connected like a voltmeter, and set to measure frequency.
When the measurement is taken, the instrument counts the signal. It might count zero crossings of the signal, or it might just assume that the signal is a sequence of pulses, and count the pulses. In either case, the instrument counts for a predetermined length of time, T (which you might be able to control).
Then, the frequency is computed by dividing the count by the time period, T.
The computation of frequency cannot have a resolution better than one count. For example, if the instrument counts for one second, a count of ten would compute as 10 Hz, and a count of 11 would compute as 11 Hz. You couldn't get a good measurement of 10.5 Hz, and would always be off by 0.5 Hz. What you got would depend upon the timing of the count - when it started.
The resolution is probably not a problem if you are interested in a 20KHz signal and the instrument counts for a second, but you have to be cognizant of what is taking place.
No comments:
Post a Comment